Microporous membranes can be used, for example, as battery separators in lithium primary batteries and secondary batteries, lithium polymer batteries, nickel-metal hydride batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc secondary batteries and the like. Various properties are required when the microporous membrane is used as a battery separator, particularly as a lithium-ion battery separator. These requirements include excellent slip characteristics due to a low friction coefficient between membranes, and high durability of the microporous membrane having a dense microporous structure. With respect to the former, a high friction coefficient causes to generate peeling charge when the membrane is unwinded from a roll state, resulting in a deterioration of handling and also in discomfort during operation due to electro-static sticking of the membrane to surrounding devices, apparatus and the like. Further, if foreign substances adhered to the membrane by static electricity are taken inside the battery, it is concerned that pinholes may occur. With respect to the latter, for example, it leads to decreasing the probability of occurrence of short-circuit, even in cases where a voltage is applied to the right surface and the back surface of the membrane within a battery. In this case, air permeability usually becomes worse due to densification of micropores, but it is desirable that the above described performance is exhibited while maintaining a certain range of air permeability. In addition, since foreign substances adhered to the surface of the membrane can also cause the occurrence of short-circuit, it is desirable that the membrane has a surface to which foreign substances are not easily adhered, that is, a surface having good slip characteristics, and has a dense micropore structure, in order to increase the breakdown voltage of the membrane.
However, there are very few prior art documents regarding the surface slip characteristics and the micropore density of microporous membranes.